How to bring application to front when the notification icon is clicked (from Service)?

I have an application that plays media with the help of a service. This runs smoothly with the help of a service ( ongoing notification is created ). When the back key is pressed the application goes to background. The problem is when I click the notification icon a new instance of my application is created every time. How can I get my application from the background to the front with the help of the notification bar?